We compared simultaneously measured canine tracheal transport rates of Dowex anion exchange particles with macroaggregates of albumin, and with sulfur colloid. We found that sulfur colloid moved significantly faster than large Dowex particles (diameter 180 micrometer), which had transport rates similar to macroaggregates of albumin, and small Dowex particles diameter 3 micrometer). We examined the alteration of airway fluid caused by intravenous 5% saline or subcutaneous methacholine chloride. We then studied the effects of the altered airway fluid on the transport rates of large Dowex particles and sulfur colloid. We found that sulfur colloid continued to be transported faster than Dowex particles. Although we do not have an explanation for the faster transport of sulfur colloid, we believe that the physiochemical properties of the marker may influence its rate of tracheal transport.